The present invention relates to a method for producing material, and, more particularly, relates to a method for producing composite material composed of a reinforcing carbon material such as carbon fibers or graphite particles embedded in a matrix metal.
There are known various types of reinforced materials, in which carbon fibers or graphite particles are embedded in a matrix metal such as aluminum or magnesium or the like to form a composite material, and these carbon/metal composite materials exhibit various excellent properties with regard to mechanical strength and wear resistance and so on which are not exhibited by either of the constituent materials individually. Accordingly the use of such composite materials has become very desirable for a range of applications. Various methods of production for such carbon/metal composite or reinforced material have already been proposed.
One such known method for producing such carbon/metal composite material is called the diffusion bonding method, or the hot pressing method. In this method, a number of sheets are made of carbon fiber and matrix metal by spraying molten matrix metal onto sheets or mats of carbon fiber in a vacuum; and then these sheets are overlaid together, again in a vacuum, and are pressed together at high temperature so that they stick together by the matrix metal diffusing between them. In this method, it is important for the carbon fibers to be well wetted by the matrix metal as it thus diffuses.
Another known method for producing such fiber reinforced material is called the infiltration method, or the autoclave method. In this method, carbon fibers are filled into a container, the carbon fibers are then evacuated of atmosphere, and then molten matrix metal is admitted into the container under pressure, so that this molten matrix metal infiltrates into the carbon fibers. This method, also, requires the use of a vacuum device for producing a vacuum, in order to provide good contact between the matrix metal and the reinforcing material at their interface, without interference caused by atmospheric air trapped in the interstices of the fiber mass. In fact, if the combination of the reinforcing material and the matrix metal has poor wettability, a good resulting fiber reinforced material cannot be obtained; and thus again it is important for the carbon fibers to be well wetted by the matrix metal as it thus infiltrates into said carbon fibers.
There is a further third method known for making carbon/metal composite material, which does not use a vacuum device. In this method, the so called high pressure casting method, after charging a mold with carbon material in the form of fiber or the like, molten matrix metal is poured into the mold and is pressurized to a high pressure exceeding 1000 kg/cm.sup.2, and this high pressure forces the molten matrix metal to infiltrate into the interstices of the reinforcing carbon material. Then the combination of the reinforcing carbon material and the matrix metal is cooled down, while still being kept under this high pressure, until all the matrix metal has completely solidified. Further, it has been conceived of to preheat the carbon material before charging the molten matrix metal into the mold. In this high pressure casting method, it is yet again important for the carbon material to be well wetted by the matrix metal as it thus diffuses.
Conventionally known techniques for thus ensuring good wettability between the carbon material and the molten matrix metal include the following process. First the reinforcing carbon material such as carbon fibers is steeped in a mixture of stearic acid and an organic titanium compound such as an ester of titanic acid, so as to cause a coating of this organic titanium compound to adhere to the surface of said reinforcing carbon material. Next either of the following two processes is performed: either (A) a coating of titanium oxide is formed on the surface of the reinforcing carbon material by heating the reinforcing carbon material with said coating of the mixture on its surface to a temperature of about 400.degree. C.; or (B) a coating of titanium carbide is formed on the surface of the reinforcing carbon material by heating the reinforcing carbon material with said coating of the mixture on its surface to a temperature of about 1200.degree. C.
This prior method, in both the forms thereof described above, has the disadvantage that, after bringing together the reinforcing carbon material and the organic compound of titanium in the presence of stearic acid, it is necessary to heat treat the reinforcing coated carbon material at a high temperature of 400.degree. C. or 1200.degree. C.; and in order to prevent oxidation degradation of the reinforcing coated carbon material at this time it is necessary to perform this heat treatment in a reducing atmosphere or in vacuum, which is very troublesome and adds to the cost of the process to a very substantial extent. Further, the choice of the proper organic titanium compound in order to improve the wettability between the reinforcing carbon material and the molten matrix metal which is to be added thereto is important, because, of course, not all of the organic compounds of titanium are effective on improvement of wettability.
Another prior art method which has been used in order to improve the wettability between the reinforcing carbon material and the molten matrix metal which is to be added thereto is as follows. In the case of distributing graphite particles or the like as a reinforcing material throughout the body of a mass of aluminum alloy or the like which is being used as a matrix metal, which has been practiced in order to improve the wear resistance of the resulting material over the wear resistance of a similar material not using graphite additive material, it has been practiced to coat the graphite particles with nickel or copper before they are dispersed in the molten matrix metal.
However, this method of improving the wettability between the reinforcing carbon material and the molten matrix metal suffers from the disadvantage that a part of this nickel or copper coating on the reinforcing carbon material diffuses into the matrix metal while the matrix metal is melted and as said matrix metal is compounded with the reinforcing carbon material. This is likely to alter the characteristics of the matrix metal and accordingly of the final carbon/metal composite material, and may significantly deteriorate the properties of the resulting material.